The present invention relates to a color cathode ray tube having an electron gun configured to project three electron beams toward a phosphor screen.
In color cathode ray tubes for use in TV receiver sets or monitors, spot shapes of the electron beams on the screen have to be properly controlled with increase in beam deflection to provide good focus and high resolution over the entire phosphor screen (also referred to merely as the screen or the picture area).
FIG. 5 is a longitudinal cross-sectional view of a color cathode ray tube for explaining its overall structure, to which the present invention is applied. Reference numeral 31 denotes a panel portion for carrying a screen, 32 is a neck portion for housing an electron gun, 33 is a funnel portion for connecting the panel portion 31 and the neck portion 32, 34 is a phosphor screen coated on the inner surface of the panel portion 31, 35 is a shadow mask serving as a color selection electrode, 36 is a mask frame for supporting the shadow mask 35, 37 is a magnetic shield for shielding external magnetic fields, 38 is springs for suspending the shadow mask 35, 39 is an electron gun for projecting three electron beams arranged in a line, 40 is a deflection yoke, 41 is a magnet assembly for centering the beams and adjusting color purity and convergence of the beams, B denotes three electron beams arranged in a line (two side beams SB and one center beam CB).
The vacuum envelope of this color cathode ray tube is formed of the panel portion 31, the neck portion 32, and the funnel portion 33 around which the deflection yoke 40 is mounted. The electron gun 39 housed in the neck portion 32 projects the three in-line beams B toward the phosphor screen 34. The deflection yoke 40 mounted around the transition region between the funnel portion 33 and the neck portion 32 generates the magnetic field for deflecting the three electron beams B from the electron gun 39 in two horizontal and vertical directions. The shadow mask 35 is welded to the mask frame 36, and the mask frame 36 is suspended within the panel portion 31 by engaging its suspension springs 38 fixed to its peripheral portions with panel pins embedded in the inner surface of the panel portion 31 such that the shadow mask is spaced a predetermined distance from the phosphor screen 34.
FIG. 6A is a vertical cross-sectional view of a three in-line beam electron gun in a color cathode ray tube for explaining a dimensional relationship of the present invention and the prior art, and FIG. 6B is a cross-sectional view taken along line VIB--VIB of FIG. 6A. Reference numeral 1 denotes a cathode structure, 2 is a beam control electrode, 3 is an accelerating electrode, 4 is a focus electrode, 5 is an anode and 6 is a shield cup. The focus electrode 4 is divided into a first focus sub-electrode 4-1 and a second focus sub-electrode 4-2. The cathode structure 1, the beam control electrode 2 and the accelerating electrode 3 constitute an electron beam generating section.
Thermoelectrons emitted from the heated cathode structure 1 are accelerated toward the beam control electrode 2 by the potential of the accelerating electrode 3 to form three electron beams. The three electron beams pass through the apertures in the beam control electrode 2, then pass through the apertures in the accelerating electrodes 3, are slightly focused by a prefocus lens formed between the accelerating electrode 3 and the first focus sub-electrode 4-1, then are accelerated and enter a main lens 7 formed between the second focus sub-electrode 4-2 and the anode 5. After they are focused by the main lens 7, they pass through apertures in the shadow mask 35 and are focused on the phosphor screen 34 to form three beam spots on the phosphor screen 34.
Four vertical parallel plate-like electrodes 411, 412, 413, 414 (only 413 is visible) and two horizontal parallel plate-like electrodes 421, 422 are attached to the first focus sub-electrode 4-1 and the second focus sub-electrode 4-2, respectively, to form an electrostatic quadrupole lens 8 therebetween.
The electrostatic quadrupole lens 8 are formed by the four vertical parallel plate-like electrodes 411, 412, 413, 414 (only 413 is visible) disposed to sandwich, in a direction of the in-line beam arrangement, respective beam apertures in the end of the first focus sub-electrode 4-1 facing the second focus sub-electrode 4-2 and electrically connected to the first focus sub-electrode 4-1, and a pair of horizontal parallel plate-like electrodes 421, 422 disposed to sandwich, in a direction perpendicular to the in-line beam arrangement, three beam apertures 4-2a, 4-2b, 4-2c in the end of the second focus sub-electrode 4-2 facing the first focus sub-electrode 4-1 and electrically connected to the second focus sub-electrode 4-2.
As shown in FIG. 7, a fixed voltage Vf1 is applied to the first focus sub-electrode 4-1, and a dynamic voltage (Vf2+dVf) varying in synchronism with deflection of the electron beams scanned on the phosphor screen 34 is applied to the second focus sub-electrode 4-2. The anode 5 is supplied with the highest voltage Eb (anode voltage).
With this structure, the strength of the main lens 7 is varied with deflection of the electron beams to correct the curvature of the image field, astigmatism is corrected by the electrostatic quadrupole lens 8 formed by the first and second focus sub-electrodes 4-1, 4-2 with deflection of the electron beams such that focus lengths of the electron beams and the shapes of the beams on the phosphor screen are controlled to provide good focus over the entire phosphor screen 34.
The electrostatic quadrupole lens 8 is configured such that the quadrupole lens is formed in a space where two horizontal parallel plate-like electrodes 421, 422 and four vertical parallel plate-like electrodes 411, 412, 413, 414 overlap each other. The strength of the quadrupole lens increases with increase in the overlapped length of the plate-like electrodes.
The above-described electrostatic quadrupole formed by the first and second sub-electrodes 4-1, 4-2 of the focus electrode 4 in an electron gun are disclosed in Japanese Patent Application Laid-Open No. Sho 61-250934, for example.
The prior art electrostatic quadrupole lens is formed by combination of plate-like electrodes, in a space where horizontal parallel plate-like electrodes and vertical parallel plate-like electrodes are spaced a relatively great distance from each other, a uniform quadrupole lens is produced in the space, but in space where they are spaced a relatively short distance, a greatly distorted quadrupole lens is generated in the space.
When the trajectory of electron beam B is bent in the electron gun due to manufacturing variations in electron guns or color cathode ray tubes, and as a result the electron beam B traverse corners off the axis of the electrostatic quadrupole lens as illustrated in FIG. 6B, the problem arises in that they are influenced by the distorted quadrupole lens action, produce greatly distorted beam spot shapes including a core C and a halo H on the phosphor screen as illustrated in FIG. 6c and deteriorate focus characteristics resulting in degradation of resolution.